1. Field of the Invention
The present invention relates to an optical encoder.
2. Description of Related Art
An optical encoder detecting a position of a detection head with respect to a scale is widely used for position detection in a measuring apparatus or the like. An incremental-type optical encoder includes a main signal scale detecting a relative position of a detection head, and an origin point detection scale detecting a position of an origin point. By taking the origin point detected by the origin point detection scale as a reference, the incremental-type optical encoder can convert relative position data detected by the main signal scale into absolute position data. Accordingly, the position of the origin point must be detected with a high degree of accuracy in the incremental-type optical encoder.
Japanese Patent Laid-open Publication No. S56-14112 describes an optical encoder using a main scale that includes a first grid portion formed at a predetermined grid pitch; and a second grid portion formed at a grid pitch that is an integer multiple of the first grid portion grid pitch. By superimposing a signal where light that has passed through the first grid portion is received and a signal where light that has passed through the second grid portion is received, the optical encoder described by Japanese Patent Laid-open Publication No. S56-14112 is able to accurately detect an origin point position.
The optical encoder described in Japanese Patent Laid-open Publication No. S56-14112 has a two grid configuration that includes a main scale; a read-out scale (photoreceiver grid) positioned directly above a photoreceiver element; and a light source emitting light at the main scale. The optical encoder having the two grid configuration detects the origin point position when the grids of the main scale and the read-out scale match. In order to correctly detect the origin point position, the main scale, read-out scale, and light source must be positioned with a high degree of accuracy in the optical encoder having the two grid configuration. Therefore, costs for manufacturing a highly accurate optical encoder having a two grid configuration are magnified and become expensive.
In addition, a light source grid having the same grid as the read-out scale can be added to the main scale, read-out scale, and light source described in Japanese Patent Laid-open Publication No. S56-14112 to yield an optical encoder having a three grid configuration. The optical encoder having the three grid configuration detects a position using an interference fringe, and therefore can detect a position with a greater degree of accuracy than the two grid configuration. However, in the optical encoder having the three grid configuration using the main scale described in Japanese Patent Laid-open Publication No. S56-14112, in addition to an interference fringe generated by the first grid portion, an interference fringe generated by the second grid portion also passes through the read-out scale and is detected by the photoreceiver element. Therefore, accuracy in detection of the origin point position may be reduced.